Does declining carbon‐use efficiency explain thermal acclimation of soil respiration with warming?

Enhanced soil respiration in response to global warming may substantially increase atmospheric CO₂ concentrations above the anthropogenic contribution, depending on the mechanisms underlying the temperature sensitivity of soil respiration. Here, we compared short‐term and seasonal responses of soil respiration to a shifting thermal environment and variable substrate availability via laboratory incubations. To analyze the data from incubations, we implemented a novel process‐based model of soil respiration in a hierarchical Bayesian framework. Our process model combined a Michaelis–Menten‐type equation of substrate availability and microbial biomass with an Arrhenius‐type nonlinear temperature response function. We tested the competing hypotheses that apparent thermal acclimation of soil respiration can be explained by depletion of labile substrates in warmed soils, or that physiological acclimation reduces respiration rates. We demonstrated that short‐term apparent acclimation can be induced by substrate depletion, but that decreasing microbial biomass carbon (MBC) is also important, and lower MBC at warmer temperatures is likely due to decreased carbon‐use efficiency (CUE). Observed seasonal acclimation of soil respiration was associated with higher CUE and lower basal respiration for summer‐ vs. winter‐collected soils. Whether the observed short‐term decrease in CUE or the seasonal acclimation of CUE with increased temperatures dominates the response to long‐term warming will have important consequences for soil organic carbon storage.     

Methane and soil CO2 production from current‐season photosynthates in a rice paddy exposed to elevated CO2 concentration and soil temperature

Quantification of rhizodeposition (root exudates and root turnover) represents a major challenge for understanding the links between above‐ground assimilation and below‐ground anoxic decomposition of organic carbon in rice paddy ecosystems. Free‐air CO₂ enrichment (FACE) fumigating depleted ¹³CO₂ in rice paddy resulted in a smaller ¹³C/¹²C ratio in plant‐assimilated carbon, providing a unique measure by which we partitioned the sources of decomposed gases (CO₂ and CH₄) into current‐season photosynthates (new C) and soil organic matter (old C). In addition, we imposed a soil‐warming treatment nested within the CO₂ treatments to assess whether the carbon source was sensitive to warming. Compared with the ambient CO₂ treatment, the FACE treatment decreased the ¹³C/¹²C ratio not only in the rice‐plant carbon but also in the soil CO₂ and CH₄. The estimated new C contribution to dissolved CO₂ was minor (ca. 20%) at the tillering stage, increased with rice growth and was about 50% from the panicle‐formation stage onwards. For CH₄, the contribution of new C was greater than for heterotrophic CO₂ production; ca. 40–60% of season‐total CH₄ production originated from new C with a tendency toward even larger new C contribution with soil warming, presumably because enhanced root decay provided substrates for greater CH₄ production. The results suggest a fast and close coupling between photosynthesis and anoxic decomposition in soil, and further indicate a positive feedback of global warming by enhanced CH₄ emission through greater rhizodeposition.     

Deep peat warming increases surface methane and carbon dioxide emissions in a black spruce‐dominated ombrotrophic bog

Boreal peatlands contain approximately 500 Pg carbon (C) in the soil, emit globally significant quantities of methane (CH₄), and are highly sensitive to climate change. Warming associated with global climate change is likely to increase the rate of the temperature‐sensitive processes that decompose stored organic carbon and release carbon dioxide (CO₂) and CH₄. Variation in the temperature sensitivity of CO₂ and CH₄ production and increased peat aerobicity due to enhanced growing‐season evapotranspiration may alter the nature of peatland trace gas emission. As CH₄ is a powerful greenhouse gas with 34 times the warming potential of CO₂, it is critical to understand how factors associated with global change will influence surface CO₂ and CH₄ fluxes. Here, we leverage the Spruce and Peatland Responses Under Changing Environments (SPRUCE) climate change manipulation experiment to understand the impact of a 0–9°C gradient in deep belowground warming (“Deep Peat Heat”, DPH) on peat surface CO₂ and CH₄ fluxes. We find that DPH treatments increased both CO₂ and CH₄ emission. Methane production was more sensitive to warming than CO₂ production, decreasing the C‐CO₂:C‐CH₄ of the respired carbon. Methane production is dominated by hydrogenotrophic methanogenesis but deep peat warming increased the δ¹³C of CH₄ suggesting an increasing contribution of acetoclastic methanogenesis to total CH₄ production with warming. Although the total quantity of C emitted from the SPRUCE Bog as CH₄ is <2%, CH₄ represents >50% of seasonal C emissions in the highest‐warming treatments when adjusted for CO₂ equivalents on a 100‐year timescale. These results suggest that warming in boreal regions may increase CH₄ emissions from peatlands and result in a positive feedback to ongoing warming.     

Application of a two‐pool model to soil carbon dynamics under elevated CO2

Elevated atmospheric CO₂ concentrations increase plant productivity and affect soil microbial communities, with possible consequences for the turnover rate of soil carbon (C) pools and feedbacks to the atmosphere. In a previous analysis (Van Groenigen et al., 2014), we used experimental data to inform a one‐pool model and showed that elevated CO₂ increases the decomposition rate of soil organic C, negating the storage potential of soil. However, a two‐pool soil model can potentially explain patterns of soil C dynamics without invoking effects of CO₂ on decomposition rates. To address this issue, we refit our data to a two‐pool soil C model. We found that CO₂ enrichment increases decomposition rates of both fast and slow C pools. In addition, elevated CO₂ decreased the carbon use efficiency of soil microbes (CUE), thereby further reducing soil C storage. These findings are consistent with numerous empirical studies and corroborate the results from our previous analysis. To facilitate understanding of C dynamics, we suggest that empirical and theoretical studies incorporate multiple soil C pools with potentially variable decomposition rates.     

Potential feedback mediated by soil microbiome response to warming in a glacier forefield

Mountain glaciers are retreating at an unprecedented rate due to global warming. Glacier retreat is widely believed to be driven by the physiochemical characteristics of glacier surfaces; however, the current knowledge of such biological drivers remains limited. An estimated 130 Tg of organic carbon (OC) is stored in mountain glaciers globally. As a result of global warming, the accelerated microbial decomposition of OC may further accelerate the melting process of mountain glaciers by heat production with the release of greenhouse gases, such as carbon dioxide (CO₂) and methane. Here, using short‐term aerobic incubation data from the forefield of Urumqi Glacier No. 1, we assessed the potential climate feedback mediated by soil microbiomes at temperatures of 5°C (control), 6.2°C (RCP 2.6), 11°C (RCP 8.5), and 15°C (extreme temperature). We observed enhanced CO₂‐C release and heat production under warming conditions, which led to an increase in near‐surface (2 m) atmospheric temperatures, ranging from 0.9°C to 3.4°C. Warming significantly changed the structures of the RNA‐derived (active) and DNA‐derived (total) soil microbiomes, and active microbes were more sensitive to increased temperatures than total microbes. Considering the positive effects of temperature and deglaciation age on the CO₂‐C release rate, the alterations in the active microbial community structure had a negative impact on the increased CO₂‐C release rate. Our results revealed that glacial melting could potentially be significantly accelerated by heat production from increased microbial decomposition of OC. This risk might be true for other high‐altitude glaciers under emerging warming, thus improving the predictions of the effects of potential feedback on global warming.     

Temperature and peat type control CO2 and CH4 production in Alaskan permafrost peats

Controls on the fate of ~277 Pg of soil organic carbon (C) stored in permafrost peatland soils remain poorly understood despite the potential for a significant positive feedback to climate change. Our objective was to quantify the temperature, moisture, organic matter, and microbial controls on soil organic carbon (SOC) losses following permafrost thaw in peat soils across Alaska. We compared the carbon dioxide (CO₂) and methane (CH₄) emissions from peat samples collected at active layer and permafrost depths when incubated aerobically and anaerobically at ?5, ?0.5, +4, and +20 °C. Temperature had a strong, positive effect on C emissions; global warming potential (GWP) was >3× larger at 20 °C than at 4 °C. Anaerobic conditions significantly reduced CO₂ emissions and GWP by 47% at 20 °C but did not have a significant effect at ?0.5 °C. Net anaerobic CH₄ production over 30 days was 7.1 ± 2.8 μg CH₄‐C gC^?1 at 20 °C. Cumulative CO₂ emissions were related to organic matter chemistry and best predicted by the relative abundance of polysaccharides and proteins (R² = 0.81) in SOC. Carbon emissions (CO₂‐C + CH₄‐C) from the active layer depth peat ranged from 77% larger to not significantly different than permafrost depths and varied depending on the peat type and peat decomposition stage rather than thermal state. Potential SOC losses with warming depend not only on the magnitude of temperature increase and hydrology but also organic matter quality, permafrost history, and vegetation dynamics, which will ultimately determine net radiative forcing due to permafrost thaw.     

Enhanced decomposition of stable soil organic carbon and microbial catabolic potentials by long‐term field warming

Quantifying soil organic carbon (SOC) decomposition under warming is critical to predict carbon–climate feedbacks. According to the substrate regulating principle, SOC decomposition would decrease as labile SOC declines under field warming, but observations of SOC decomposition under warming do not always support this prediction. This discrepancy could result from varying changes in SOC components and soil microbial communities under warming. This study aimed to determine the decomposition of SOC components with different turnover times after subjected to long‐term field warming and/or root exclusion to limit C input, and to test whether SOC decomposition is driven by substrate lability under warming. Taking advantage of a 12‐year field warming experiment in a prairie, we assessed the decomposition of SOC components by incubating soils from control and warmed plots, with and without root exclusion for 3 years. We assayed SOC decomposition from these incubations by combining inverse modeling and microbial functional genes during decomposition with a metagenomic technique (GeoChip). The decomposition of SOC components with turnover times of years and decades, which contributed to 95% of total cumulative CO₂ respiration, was greater in soils from warmed plots. But the decomposition of labile SOC was similar in warmed plots compared to the control. The diversity of C‐degradation microbial genes generally declined with time during the incubation in all treatments, suggesting shifts of microbial functional groups as substrate composition was changing. Compared to the control, soils from warmed plots showed significant increase in the signal intensities of microbial genes involved in degrading complex organic compounds, implying enhanced potential abilities of microbial catabolism. These are likely responsible for accelerated decomposition of SOC components with slow turnover rates. Overall, the shifted microbial community induced by long‐term warming accelerates the decomposition of SOC components with slow turnover rates and thus amplify the positive feedback to climate change.     

Temperature sensitivity of greenhouse gas production in wetland soils of different vegetation

Organic matter decomposition regulates rates of carbon loss (CO₂ and CH₄) in wetlands and has implications for carbon sequestration in the context of changing global temperature. Here we determined the influence of temperature and vegetation type on both aerobic and anaerobic decomposition of organic matter in subtropical wetland soils. As in many other studies, increased temperature resulted in higher rates of respiration and methanogenesis under both aerobic and anaerobic conditions, and the positive effect of temperature depended on vegetation (source of carbon substrate to soil). Under anaerobic incubations, the proportion of gaseous C (CO₂ and CH₄) lost as CH₄ increased with temperature indicating a greater sensitivity of methanogenesis to temperature. This was further supported by a wider range of Q₁₀ values (1.4–3.6) for methane production as compared with anaerobic CO₂ (1.3–2.5) or aerobic CO₂ (1.4–2.1) production. The increasing strength of positive linear correlation between CO₂:CH₄ ratio and the soil organic matter ligno-cellulose index at higher temperature indicated that the temperature sensitivity of methanogenesis was likely the result of increased C availability at higher temperature. This information adds to our basic understanding of decomposition in warmer subtropical and tropical wetland systems and has implications for C models in wetlands with different vegetation types.     

Pathways regulating decreased soil respiration with warming in a biocrust‐dominated dryland

A positive soil carbon (C)‐climate feedback is embedded into the climatic models of the IPCC. However, recent global syntheses indicate that the temperature sensitivity of soil respiration (R_S) in drylands, the largest biome on Earth, is actually lower in warmed than in control plots. Consequently, soil C losses with future warming are expected to be low compared with other biomes. Nevertheless, the empirical basis for these global extrapolations is still poor in drylands, due to the low number of field experiments testing the pathways behind the long‐term responses of soil respiration (R_S) to warming. Importantly, global drylands are covered with biocrusts (communities formed by bryophytes, lichens, cyanobacteria, fungi, and bacteria), and thus, R_S responses to warming may be driven by both autotrophic and heterotrophic pathways. Here, we evaluated the effects of 8‐year experimental warming on R_S, and the different pathways involved, in a biocrust‐dominated dryland in southern Spain. We also assessed the overall impacts on soil organic C (SOC) accumulation over time. Across the years and biocrust cover levels, warming reduced R_S by 0.30 μmol CO₂ m^?2 s^?1 (95% CI = ?0.24 to 0.84), although the negative warming effects were only significant after 3 years of elevated temperatures in areas with low initial biocrust cover. We found support for different pathways regulating the warming‐induced reduction in R_S at areas with low (microbial thermal acclimation via reduced soil mass‐specific respiration and β‐glucosidase enzymatic activity) vs. high (microbial thermal acclimation jointly with a reduction in autotrophic respiration from decreased lichen cover) initial biocrust cover. Our 8‐year experimental study shows a reduction in soil respiration with warming and highlights that biocrusts should be explicitly included in modeling efforts aimed to quantify the soil C–climate feedback in drylands.     

Increased soil release of greenhouse gases shrinks terrestrial carbon uptake enhancement under warming

Warming can accelerate the decomposition of soil organic matter and stimulate the release of soil greenhouse gases (GHGs), but to what extent soil release of methane (CH₄) and nitrous oxide (N₂O) may contribute to soil C loss for driving climate change under warming remains unresolved. By synthesizing 1,845 measurements from 164 peer‐reviewed publications, we show that around 1.5°C (1.16–2.01°C) of experimental warming significantly stimulates soil respiration by 12.9%, N₂O emissions by 35.2%, CH₄ emissions by 23.4% from rice paddies, and by 37.5% from natural wetlands. Rising temperature increases CH₄ uptake of upland soils by 13.8%. Warming‐enhanced emission of soil CH₄ and N₂O corresponds to an overall source strength of 1.19, 1.84, and 3.12 Pg CO₂‐equivalent/year under 1°C, 1.5°C, and 2°C warming scenarios, respectively, interacting with soil C loss of 1.60 Pg CO₂/year in terms of contribution to climate change. The warming‐induced rise in soil CH₄ and N₂O emissions (1.84 Pg CO₂‐equivalent/year) could reduce mitigation potential of terrestrial net ecosystem production by 8.3% (NEP, 22.25 Pg CO₂/year) under warming. Soil respiration and CH₄ release are intensified following the mean warming threshold of 1.5°C scenario, as compared to soil CH₄ uptake and N₂O release with a reduced and less positive response, respectively. Soil C loss increases to a larger extent under soil warming than under canopy air warming. Warming‐raised emission of soil GHG increases with the intensity of temperature rise but decreases with the extension of experimental duration. This synthesis takes the lead to quantify the ecosystem C and N cycling in response to warming and advances our capacity to predict terrestrial feedback to climate change under projected warming scenarios.     

Climate change feedbacks to microbial decomposition in boreal soils

Boreal ecosystems store 10–20 % of global soil carbon and may warm by 4–7 °C over the next century. Higher temperatures could increase the activity of boreal decomposers and indirectly affect decomposition through other ecosystem feedbacks. For example, permafrost melting will likely alleviate constraints on microbial decomposition and lead to greater soil CO₂ emissions. However, wet boreal ecosystems underlain by permafrost are often CH₄ sources, and permafrost thaw could ultimately result in drier soils that consume CH₄, thereby offsetting some of the greenhouse warming potential of soil CO₂ emissions. Climate change is also likely to increase winter precipitation and snow depth in boreal regions, which may stimulate decomposition by moderating soil temperatures under the snowpack. As temperatures and evapotranspiration increase in the boreal zone, fires may become more frequent, leading to additional permafrost loss from burned ecosystems. Although post-fire decomposition could also increase due to higher soil temperatures, reductions in microbial biomass and activity may attenuate this response. Other feedbacks such as soil drying, increased nutrient mineralization, and plant species shifts are either weak or uncertain. We conclude that strong positive feedbacks to decomposition will likely depend on permafrost thaw, and that climate feedbacks will probably be weak or negative in boreal ecosystems without permafrost. However, warming manipulations should be conducted in a broader range of boreal systems to validate these predictions.     

Physiological acclimation dampens initial effects of elevated temperature and atmospheric CO2 concentration in mature boreal Norway spruce

Physiological processes of terrestrial plants regulate the land–atmosphere exchange of carbon, water, and energy, yet few studies have explored the acclimation responses of mature boreal conifer trees to climate change. Here we explored the acclimation responses of photosynthesis, respiration, and stomatal conductance to elevated temperature and/or CO₂ concentration ([CO₂]) in a 3‐year field experiment with mature boreal Norway spruce. We found that elevated [CO₂] decreased photosynthetic carboxylation capacity (?23% at 25 °C) and increased shoot respiration (+64% at 15 °C), while warming had no significant effects. Shoot respiration, but not photosynthetic capacity, exhibited seasonal acclimation. Stomatal conductance at light saturation and a vapour pressure deficit of 1 kPa was unaffected by elevated [CO₂] but significantly decreased (?27%) by warming, and the ratio of intercellular to ambient [CO₂] was enhanced (+17%) by elevated [CO₂] and decreased (?12%) by warming. Many of these responses differ from those typically observed in temperate tree species. Our results show that long‐term physiological acclimation dampens the initial stimulation of plant net carbon assimilation to elevated [CO₂], and of plant water use to warming. Models that do not account for these responses may thus overestimate the impacts of climate change on future boreal vegetation–atmosphere interactions.     

Apparent thermal acclimation of soil heterotrophic respiration mainly mediated by substrate availability

Multiple lines of existing evidence suggest that increasing CO₂ emission from soils in response to rising temperature could accelerate global warming. However, in experimental studies, the initial positive response of soil heterotrophic respiration (R_H) to warming often weakens over time (referred to apparent thermal acclimation). If the decreased R_H is driven by thermal adaptation of soil microbial community, the potential for soil carbon (C) losses would be reduced substantially. In the meanwhile, the response could equally be caused by substrate depletion, and would then reflect the gradual loss of soil C. To address uncertainties regarding the causes of apparent thermal acclimation, we carried out sterilization and inoculation experiments using the soil samples from an alpine meadow with 6 years of warming and nitrogen (N) addition. We demonstrate that substrate depletion, rather than microbial adaptation, determined the response of R_H to long-term warming. Furthermore, N addition appeared to alleviate the apparent acclimation of R_H to warming. Our study provides strong empirical support for substrate availability being the cause of the apparent acclimation of soil microbial respiration to temperature. Thus, these mechanistic insights could facilitate efforts of biogeochemical modeling to accurately project soil C stocks in the future climate.     

Increasing microbial carbon use efficiency with warming predicts soil heterotrophic respiration globally

The degree to which climate warming will stimulate soil organic carbon (SOC) losses via heterotrophic respiration remains uncertain, in part because different or even opposite microbial physiology and temperature relationships have been proposed in SOC models. We incorporated competing microbial carbon use efficiency (CUE)–mean annual temperature (MAT) and enzyme kinetic–MAT relationships into SOC models, and compared the simulated mass‐specific soil heterotrophic respiration rates with multiple published datasets of measured respiration. The measured data included 110 dryland soils globally distributed and two continental to global‐scale cross‐biome datasets. Model–data comparisons suggested that a positive CUE–MAT relationship best predicts the measured mass‐specific soil heterotrophic respiration rates in soils distributed globally. These results are robust when considering models of increasing complexity and competing mechanisms driving soil heterotrophic respiration–MAT relationships (e.g., carbon substrate availability). Our findings suggest that a warmer climate selects for microbial communities with higher CUE, as opposed to the often hypothesized reductions in CUE by warming based on soil laboratory assays. Our results help to build the impetus for, and confidence in, including microbial mechanisms in soil biogeochemical models used to forecast changes in global soil carbon stocks in response to warming.     

Short- and long-term greenhouse gas and radiative forcing impacts of changing water management in Asian rice paddies

Fertilized rice paddy soils emit methane while flooded, emit nitrous oxide during flooding and draining transitions, and can be a source or sink of carbon dioxide. Changing water management of rice paddies can affect net emissions of all three of these greenhouse gases. We used denitrification–decomposition (DNDC), a process‐based biogeochemistry model, to evaluate the annual emissions of CH₄, N₂O, and CO₂ for continuously flooded, single‐, double‐, and triple‐cropped rice (three baseline scenarios), and in further simulations, the change in emissions with changing water management to midseason draining of the paddies, and to alternating crops of midseason drained rice and upland crops (two alternatives for each baseline scenario). We used a set of first‐order atmospheric models to track the atmospheric burden of each gas over 500 years. We evaluated the dynamics of the radiative forcing due to the changes in emissions of CH₄, N₂O, and CO₂ (alternative minus baseline), and compared these with standard calculations of CO₂‐equivalent emissions using global warming potentials (GWPs). All alternative scenarios had lower CH₄ emissions and higher N₂O emissions than their corresponding baseline cases, and all but one sequestered carbon in the soil more slowly. Because of differences in emissions, in radiative forcing per molecule, and in atmospheric time constants (lifetimes), the relative radiative impacts of CH₄, N₂O, and CO₂ varied over the 500‐year simulations. In three of the six cases, the initial change in radiative forcing was dominated by reduced CH₄ emissions (i.e. a cooling for the first few decades); in five of the six cases, the long‐term radiative forcing was dominated by increased N₂O emissions (i.e. a warming over several centuries). The overall complexity of the radiative forcing response to changing water management could not easily be captured with conventional GWP calculations.     

Carbon preservation in humic lakes; a hierarchical regulatory pathway

Peatland catchments store vast amounts of carbon. Humic lakes and pools are the primary receptacles for terrigenous carbon in these meta‐ecosystems, representing sequestration hotspots; boreal lakes alone store ca. 120 Pg C. But little is known about the mechanisms that preserve aquatic carbon stocks. Here, we determined the regulatory pathway of decomposition in relation to ‘traditional’ limitations, namely anoxia, decay inhibiting compounds, low nutrients and acidity, using in vitro manipulation, mesocosms and natural gradients. We show that anoxia represents a powerful hierarchical preservation mechanism affecting all major limitations on decomposition and recapturing carbon that would otherwise escape from peatlands. Oxygen constraints on microbial synthesis of oxidases and nutrient‐cycling enzymes, prevents the decay of organic matter to CO₂, CH₄ and N₂O by allowing inhibitor accumulation and lowering nutrients. However, this pathway is sensitive to direct nutrient inputs and therefore eutrophication could initiate catastrophic feedback to global warming via dramatically increased greenhouse gas emissions. Identifying these process‐specific limitations should inform better management and conservation of these vital systems.     

湿地微生物介导的甲烷排放机制

湿地生态系统是陆地上巨大的有机碳库,同时也是大气中甲烷(CH_4)的主要排放源。由于CH_4对全球的增温潜能是CO2的34倍,因此关于湿地CH_4排放在全球气候变化中有关碳汇、碳源的研究具有极其重要的意义。全球80%–90%的CH_4排放离不开微生物活动,湿地生态系统中产CH_4菌和CH_4氧化菌的种类组成、数量及功能与CH_4通量密切相关,但基于湿地生态系统中介导CH_4循环的功能微生物对甲烷排放通量的影响及作用机制研究相对比较分散。为更好地认识微生物介导的CH_4排放过程的微生物调控机制,本文综述了湿地生态系统中参与CH_4循环的功能微生物,对介导CH_4循环相关微生物活性的影响因子进行了回顾,重点总结了湿地生态系统微生物介导的CH_4排放机制,并对未来的相关研究方向进行了展望。由于湿地微生物介导的碳循环过程也可能决定了湿地生态系统对全球气候变暖的反馈,因此本文也能为全球气候变化研究提供微生物方面的参考。     

Soil carbon storage under simulated climate change is mediated by plant functional type

The stability of soil organic matter (SOM) pools exposed to elevated CO₂ and warming has not been evaluated adequately in long‐term experiments and represents a substantial source of uncertainty in predicting ecosystem feedbacks to climate change. We conducted a 6‐year experiment combining free‐air CO₂ enrichment (FACE, 550 ppm) and warming (+2 °C) to evaluate changes in SOM accumulation in native Australian grassland. In this system, competitive interactions appear to favor C₄ over C₃ species under FACE and warming. We therefore investigated the role of plant functional type (FT) on biomass and SOM responses to the long‐term treatments by carefully sampling soil under patches of C₃‐ and C₄‐dominated vegetation. We used physical fractionation to quantify particulate organic matter (POM) and long‐term incubation to assess potential decomposition rates. Aboveground production of C₄ grasses increased in response to FACE, but total root biomass declined. Across treatments, C : N ratios were higher in leaves, roots and POM of C₄ vegetation. CO₂ and temperature treatments interacted with FT to influence SOM, and especially POM, such that soil carbon was increased by warming under C₄ vegetation, but not in combination with elevated CO₂. Potential decomposition rates increased in response to FACE and decreased with warming, possibly owing to treatment effects on soil moisture and microbial community composition. Decomposition was also inversely correlated with root N concentration, suggesting increased microbial demand for older, N‐rich SOM in treatments with low root N inputs. This research suggests that C₃–C₄ vegetation responses to future climate conditions will strongly influence SOM storage in temperate grasslands.     

Positive climate feedbacks of soil microbial communities in a semi‐arid grassland

Soil microbial communities may be able to rapidly respond to changing environments in ways that change community structure and functioning, which could affect climate–carbon feedbacks. However, detecting microbial feedbacks to elevated CO₂ (eCO₂) or warming is hampered by concurrent changes in substrate availability and plant responses. Whether microbial communities can persistently feed back to climate change is still unknown. We overcame this problem by collecting microbial inocula at subfreezing conditions under eCO₂ and warming treatments in a semi‐arid grassland field experiment. The inoculant was incubated in a sterilised soil medium at constant conditions for 30 days. Microbes from eCO₂ exhibited an increased ability to decompose soil organic matter (SOM) compared with those from ambient CO₂ plots, and microbes from warmed plots exhibited increased thermal sensitivity for respiration. Microbes from the combined eCO₂ and warming plots had consistently enhanced microbial decomposition activity and thermal sensitivity. These persistent positive feedbacks of soil microbial communities to eCO₂ and warming may therefore stimulate soil C loss.     

Regulation of Decomposition and Methane Dynamics across Natural,Commercially Mined,and Restored Northern Peatlands

Abstract We examined aerobic and anaerobic microbial carbon dioxide (CO₂) and methane (CH₄) exchange in peat samples representing different profiles at natural, mined, mined-abandoned, and restored northern peatlands and characterized the nutrient and substrate chemistry and microbial biomass of these soils. Mining and abandonment led to reduced nutrient and substrate availability and occasionally drier conditions in surface peat resulting in a drastic reduction in CO₂ and CH₄ production, in agreement with previous studies. Owing mainly to wetter conditions, CH₄ production and oxidation were faster in restored block-cut than natural sites, whereas in one restored site, increased substrate and nutrient availability led to much more rapid rates of CO₂ production. Our work in restored block-cut sites compliments that in vacuum-harvested peatlands undergoing more recent active restoration attempts. The sites we examined covered a large range of soil C substrate quality, nutrient availability, microbial biomass, and microbial activities, allowing us to draw general conclusions about controls on microbial CO₂ and CH₄ dynamics using stepwise regression analysis among all sites and soil depths. Aerobic and anaerobic decomposition of peat was constrained by organic matter quality, particularly phosphorus (P) and carbon (C) chemistry, and closely linked to the size of the microbial biomass supported by these limiting resources. Methane production was more dominantly controlled by field moisture content (a proxy for anaerobism), even after 20 days of anaerobic laboratory incubation, and to a lesser extent by C substrate availability. As methanogens likely represented only a small proportion of the total microbial biomass, there were no links between total microbial biomass and CH₄ production. Methane oxidation was controlled by the same factors influencing CH₄ production, leading to the conclusion that CH₄ oxidation is primarily controlled by substrate (that is, CH₄) availability. Although restoring hydrology similar to natural sites may re-establish CH₄ dynamics, there is geographic or site-specific variability in the ability to restore peat decomposition dynamics.